1. Technical Field
The present invention relates to multilayer printed circuit boards. More particularly, the present invention relates to the use of cyanate ester laminates to produce multilayer printed circuit boards.
2. Description of the Prior Art
Multilayer printed circuit boards are manufactured from various resins, which are mixed with solvents and other chemicals, and then coated onto a glass fabric. The resulting board material is available in a partially cured form, referred to `prepreg`, and in a cured form, usually having a metal coating on one or both sides, and which is referred to as a `core`.
The cores are patterned, typically using an image etch process, to produce an electrical interconnect on the metal coated surface of each core. The patterned cores, each of which makes up one or two layers of a multilayer printed circuit board, are then stacked. One or more layers of prepreg material are placed between each core in the stack. The stacked materials are then cured to form a multilayer laminate.
The curing process involves the application of heat and pressure using various techniques: for example, press molding, with or without vacuum assist; and autoclave molding, with or without vacuum bags. The FR-4 materials are laminated over a very wide range of pressures, e.g. 25-450 psi, based on economic convenience, but more typically in the 200-350 psi range. Manufacturers may adjust the heat rise and pressure to affect cycle times in a variety of beneficial ways. The laminating temperature varies according to the degree of cure needed to produce physical and mechanical properties required for the end product. A typical temperature of 180.degree. C. is produced by gradually raising the temperature, holding the temperature constant for a period of time, and then lowering the temperature to room temperature.
After curing, the boards are drilled, imaged and plated to electrically connect surface and internal conductive layers, and components are mounted to the board, by wave soldering or surface mount techniques.
Of the various resins available, FR-4, has been the most commonly used and widely accepted material for making printing circuit boards. Other resins that are in use in the printed circuit board industry include polyimides, BT epoxy, and cyanate esters, among yet others. Polyimides, due to their high cost, have found limited acceptance; while BT epoxy is difficult to process and therefore expensive.
Cyanate esters have several advantages over both FR-4 and polyimides and are therefore an attractive alternative to these and other materials. Cyanate esters are less expensive than polyimides; they are better suited for thick multilayer structures than FR-4 because they have a low thermal coefficient of expansion; and they are better suited for high frequency applications than FR-4 because they have a lower dielectric constant and low loss factors. Thus, in high frequency applications, e.g. 50-100 MHz, printed circuit boards made of FR-4 require special design considerations, including component placement, and load calculations to take into account the RF characteristics of the board. Additionally, it is very difficult to make thick boards of FR-4 because the vias used to interconnect the various layers of a multilayer FR-4 printed circuit board are subjected to considerable stress due to expansion of the board under conditions of thermal cycling, such that the vias often fail.
Cyanate ester resins, such as Arocy B-40S, are available from chemical manufacturers, such as Ciba Geigy of Switzerland, among others; cores and prepreg materials made from cyanate ester resins, such as N8000, are available from laminators, such as Nelco of Fullerton, Calif., among others.
One of the major drawbacks of cyanate ester printed circuit board laminates has been the tendency of such laminates to form microcracks during normal (i.e. FR-4 type) board fabrication processes. Microcracks form around the drilled holes in a printed circuit board and sometimes on the laminate surface when the cyanate ester material comes into contact with caustic solutions as are typically used in processing printed circuit boards. Such caustic solutions are used for a variety of purposes, e.g. electroless Cu plating, image resist development, and de-smearing of drilled holes.
These microcracks can cause long term electrical reliability problems on finished assemblies in products. For example, the cracks allow electrolytes to accumulate within the board material which, in time, may grow dendrites that become metal connections, and, that ultimately create short circuits. This process is referred to as anodic cathodic filamentosis. It is important to note that the cracks form in the cyanate ester polymer resin in pure and alloyed forms of the material (this includes catalysts, flame retardants, and solvents), with and without glass reinforcement.